I’m talking about the insanely bright, flashing headlights that are now standard equipment for bike commuters. During rush hour in bike-heavy Seattle where I live it is impossible to ride more than a few minutes at a stretch without getting blasted with another retina-shocking dose of flashing light from another biker heading in the opposite direction. I’m sure it’s the same in your city.

Flashing headlights are, of course, employed in the name of safety. Biking in the dark is incredibly dangerous if nobody can see you. But there is a line between personal comfort and the comfort and safety of the other people on the road. And when set on flashing mode, the current generation of overly powerful headlights is well over that line.

Once upon a time bike commuters could choose between affordable, but fairly low-powered and self-contained blinky lights that would help them be seen. Or they could get expensive lights with external battery packs that had enough power to light up the road. Just seven years ago I needed a light powerful enough to guide my daily commute on an unlit section of bike path. It cost $100, had an external battery pack and put out 110 lumens. (Lumens are one of the standard units by which light is measured. For comparison, a standard halogen car headlamp puts out about 700 lumens on low beam and 1,200 on high beam.)

As bulb and battery technology improved, the bike light industry engaged in an arms race to pack the most lumens in the smallest package for the lowest cost. Today, you can get 100 lumens in a small, self-contained unit for just $25. For around $100, you can get upwards of 1,600 lumens, enough to turn night into day in the path of your headlight!

And though many bike commuters are now riding with headlights on par with a car’s headlights, there is a critical difference. Car headlight beams are focused and directed down toward the road to ensure they don’t blind other drivers. Typically attached to handlebars and helmets, bike headlights point out and up and have a tendency to shine into the eyes of passersby. Add in flashing and it’s a recipe for irritation at best and danger at worst.

There is, of course, little question that flashing lights can help you get noticed. A 2001 study by Rensselaer Polytechnic Institute’s Lighting Research Center found that flashing lights on the rear of a snowplow increased the snowplow’s conspicuity. (It turns out drivers frequently crash into the back of snowplows). In 2008, the Dutch Ministry of Transport, Public Works and Water Management published a study of flashing and steady bike lights. They found that, “In most cases flicker raises the conspicuity of front lights.” For rear bike lights they actually found that flashing, “does not contribute significantly to the conspicuity.”

But though flashing lights may improve your chances of getting noticed, both studies found that they also make it harder for other road users to judge your speed and distance. In other words, the driver who thinks he’s got plenty of time to take a left at the intersection might hit the oncoming cyclist in the opposite lane because she looks slower and farther away than she is.

In addition to confusing drivers of your whereabouts, flashing bike lights have been shown to have a negative impact on people with photosensitive epilepsy. It is hard to find documented cases of bike lights causing seizures. But according to the Intractable Childhood Epilepsy Alliance, “There have been cases where red flashing lights (red light emitting diodes) on the back of bicycles have triggered seizures in people with photosensitive epilepsy, when they have been close to the lights as they were setting them up,” though it doesn’t cite specific incidents. Halley Weaver, a Portland cyclist with photosensitive epilepsy says that flashing bike lights make her nauseous.

In addition to all that, flashing bike lights make for a terrible experience for other bicyclists unfortunate enough to be nearby. Again, at best it’s merely annoying. Anyone who’s sat at a red light on a popular bike route can attest to the fact that it’s frustrating and uncomfortable to have a powerful flasher going off right in front of you as you wait for the light to turn green. At worst it can be dangerous. When a rider with a flashing front light passes by going the opposite direction on a dimly lit or unlit street or bike path it leaves you temporarily blinded and vulnerable to hitting hidden objects and potholes in your path. Of course this can happen with bright, steady lights, but flashing exacerbates the issues.

I give my bicycling compatriots the benefit of the doubt. The vast majority of bike lights come with a flashing mode, and it’s intuitive to think flashing mode makes you more visible to drivers and therefore safer, so why wouldn’t you put it on flash? There is little regulation of the issue. Washington State explicitly outlaws flashing front lights. To my knowledge it is the only place in the U.S. to do so, and obviously the law does not get enforced.

With all that said, flashing mode does sometimes have its place. I use flashing lights during the day when it’s particularly overcast and gray (aka, nine months of the year in Seattle) or when riding through a daytime rainstorm (again, frequently around here). There’s probably a case for using flash mode on the cheapest, lowest-powered — like 15 lumens or so — bike lights as well.

But using flashing bike lights at night doesn’t make you safer, can create medical problems for people, can be dangerous for other bicyclists and is definitely obnoxious to be around. So really, why on earth would you do it?